The increasing demand for high-performance lithium-ion batteries (LIBs) in electric vehicles (EVs) and renewable energy storage systems underscores the need for advanced cathode materials with enhanced energy density, thermal stability, and long-term cycling performance. Nickel-rich cathodes, such as NCM-622, offer high capacity and energy density but suffer from structural degradation, transition metal dissolution, and electrolyte decomposition at elevated voltages and temperatures. In this study, magnesium-doped single-crystalline (SC) NCM-622 cathode material is synthesised using a multi-step annealing process to address these challenges. The Mg2+ doping significantly improves structural stability by suppressing cation mixing, stabilizing the layered structure, and mitigating abrupt lattice distortions. The SC morphology eliminates grain boundary-induced failures, enhancing electrochemical performance and thermal stability. Electrochemical analyses reveal that the Mg-doped SC NCM-622 cathode exhibits superior cyclic stability, retaining 81.7 % of its capacity after 300 cycles at 4.3 V and maintaining performance even at elevated temperatures. This study highlights the effectiveness of Mg2+ doping and SC architecture in addressing the limitations of Ni-rich cathodes, offering a promising approach for developing high-energy-density LIBs for next-generation energy applications.